The present invention relates generally to overvoltage surge arresters for use at relatively high voltages and particularly to, but not necessarily exclusively to, such arresters for use with direct current power systems.
Overvoltage surge arresters protect the insulation of an electrical system from exposure to excessive voltages by selectively grounding the system to drain off current in such a way that an overvoltage surge in the system becomes attenuated. As electrical systems become more complex and more costly, the performance characteristics of such arresters become increasingly important. This is especially true for direct current systems which can, as one of their advantages, minimize the amount of insulation required for the components.
An arrester for high voltage operation typically includes both control gap units and cascade gap units. The control gap units differ from the cascade gap units primarily in that they have both a substantially lower sparkover voltage level and also are more accurately calibrated for their sparkover level and over a wider frequency range. Both the lower sparkover and the accuracy can be provided, for instance, by a preionizer and triggering gap associated with the power gap of the unit. The power gaps of the cascade gap units, on the other hand, typically have associated with them only a simple preionizer and are designed to have optimum sparkover characteristics for the relatively well defined internal impulse of the arrester. Thus, the control gap units always spark over before any of the cascade gap units spark over, and any one of the control gaps can be the first gap to trigger a sparkover when the arrester is subjected to a surge.
An increasingly important characteristic of arresters is the response time of the arrester to the steep wave front of a fast overvoltage impulse, such as might be generated by a lightning stroke directly to a component of the system. If the response of the arrester is not sufficiently rapid, the impulse will cause damage to insulation in the system before the arrester gaps have sparked over. The first control gap unit to spark over largely determines the response to an impulse.
A persistent problem with shortening the sparkover time of an arrester has been that in general, the sparkover voltage of a given gap is not completely independent of the time of response to the voltage. Thus, if a control gap unit having a certain sparkover voltage for a relatively slow impulse, such as a switching surge, is subjected to the steep front of a fast voltage impulse, the sparkover of the control gap unit will be at a somewhat higher voltage. This characteristic called impulse turn-up, can result in insulation damage to the system.